linux-kernel - Re: [PATCH 1/2] mm, memory_hotplug: is_mem_section

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Message-ID: <20190129091224.GG18811@dhcp22.suse.cz>
Date:   Tue, 29 Jan 2019 10:12:24 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Mikhail Zaslonko <zaslonko@...ux.ibm.com>,
        Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        gerald.schaefer@...ibm.com, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm, memory_hotplug: is_mem_section_removable do not
 pass the end of a zone

On Tue 29-01-19 10:06:05, Oscar Salvador wrote:
> On Mon, Jan 28, 2019 at 03:45:05PM +0100, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > Mikhail has reported the following VM_BUG_ON triggered when reading
> > sysfs removable state of a memory block:
> >  page:000003d082008000 is uninitialized and poisoned
> >  page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
> >  Call Trace:
> >  ([<0000000000385b26>] test_pages_in_a_zone+0xde/0x160)
> >   [<00000000008f15c4>] show_valid_zones+0x5c/0x190
> >   [<00000000008cf9c4>] dev_attr_show+0x34/0x70
> >   [<0000000000463ad0>] sysfs_kf_seq_show+0xc8/0x148
> >   [<00000000003e4194>] seq_read+0x204/0x480
> >   [<00000000003b53ea>] __vfs_read+0x32/0x178
> >   [<00000000003b55b2>] vfs_read+0x82/0x138
> >   [<00000000003b5be2>] ksys_read+0x5a/0xb0
> >   [<0000000000b86ba0>] system_call+0xdc/0x2d8
> >  Last Breaking-Event-Address:
> >   [<0000000000385b26>] test_pages_in_a_zone+0xde/0x160
> >  Kernel panic - not syncing: Fatal exception: panic_on_oops
> > 
> > The reason is that the memory block spans the zone boundary and we are
> > stumbling over an unitialized struct page. Fix this by enforcing zone
> > range in is_mem_section_removable so that we never run away from a
> > zone.
> 
> Does that mean that the remaining pages(escaping from the current zone) are not tied to
> any other zone? Why? Are these pages "holes" or how that came to be?

Yes, those pages should be unreachable because they are out of the zone.
Reasons might be various. The memory range is not mem section aligned,
or cut due to mem parameter etc.

-- 
Michal Hocko
SUSE Labs